She cheated someone with no job, homeless, shorter and younger than me, and really nasty teeth... Why?

I'll admit, I don't have the highest pay wage ($13.50 hr) and I'm not the strongest person either. But I pay all my bills and have no problem doing so. I guess I'd understand this better if she ran off with someone rich, or tall, or muscular... but wtf?

We were only together for 6 mo, but I figured it was going well. Neither of us smoke or drink, and stay away from drugs. We were actually just starting to talk about getting more serious together. But over the course of the past couple weeks she started getting really distant to the point where she stopped replying to me at all for days. Then I found out she cheated on me. It was over and everything after that but then I found out who it was...

Some 19 yr old, scawny, a good 4" shorter than me, really yellow and rotting teeth, and to boot no job and always looking for a place to stay the night because he doesn't even have a home.

I mean, I've been cheated on in most of my past relationships and I usually know by now how to cope with it, butI cannot wrap my head around this one. Anyone have any ideas?